Since Android 3 it is no longer possible to access the phone via USB mount. The reason being that Android puts everything into a single partition. As this single partition is always mounted by Android it is not easy to have two users (Android system and PC user) to write on this single partition. Therefore one needs a separate way to access the whole or parts of the filesystem. One solution is ssh, see ssh and rsync for Android, or MTPFS.
apt-get install mtpfs mtpfs /mnt/sd # mount ... # use it fusermount -u /mnt/sd # umount